Galactic coordinates and projections of the celestial sphere onto a plane

The complexities of defining the celestial system of coordinates go far beyond what is needed in this text. These arise because the Earth does not move in a perfectly elliptical orbit about the Sun but is subject to wobbles and precessions because of the perturbing influence of the Moon and planets. These issues are dealt with in the textbooks by Smart and Murray (Smart, 1977; Murray, 1983).

The positions of celestial objects are described by a fixed set of spherical polar coordinates on the sky known as right ascension (RA or α) and declination (Dec or δ). The north celestial pole (NCP) is defined to be the mean direction of the rotation axis of the Earth and declination is the polar angle measured from the equator (δ = 0°) towards the north celestial pole (δ = 90°) (Fig. A.1). The south celestial pole (SCP) has declination δ = −90°. In the year 2000.0, the Earth's rotation axis was tilted at an angle of 23° 26′ 21.448″ with respect to the direction perpendicular to the plane of the ecliptic, which is the plane of the Earth's orbit about the Sun (Fig. A.1). The coordinates of right ascension and declination are referred to the reference epoch 2000.0 which is known as the 2000.0 coordinate system. The Earth's rotation axis points more or less in the same direction as the Earth moves round the Sun and this gives rise to the seasonal changes of climate.